Wafers are automatically moved by robots both in the fabrication and during the process inspection. Some fabrication and process steps require the rotation of the wafers about their own axis. One example is the inspection of the wafers for contamination and damage by means of optical methods, such as are described in DE 102 37 477 B4. The optical measuring system has a scattered light inspection device with which the surface of the rapidly rotating wafer is scanned in spiral fashion by a laser. The above-mentioned holding device is used in order to hold the wafer and place it in rotation.
Caution in the design of such a holding system is always mandated in terms of a possible contamination. Also for this reason one must avoid any contact with the wafer surface. This applies in particular to the holding device, which if possible should not touch the wafer either on the front side or the back side. In modern semiconductor fabrication, what is more, there is an increasing interest in also inspecting the back side of the wafer, which must therefore be freely accessible there by means of a measurement system. The systems proposed for this purpose are based on various principles, as will become evident from the following discussion of the prior art in detail. Moreover, rapid rotations of several thousand revolutions per minute are desirable. It is important that the wafer run in as plane and circular a manner as possible. A deformation or vibration of the wafer would result in an impairment of the measurement result in the optical sensing method. The holding device should satisfy all these requirements.
U.S. Pat. No. 6,559,938 B1 discloses a holding device which uses a cushion of air, i.e., a fluid flow, for the distance positioning of the wafer above a table. The torque for the rotation is introduced by several driven frictional rollers arranged along a peripheral segment of the wafer, after the table with the inserted wafer has been tilted from the horizontal so that it comes to lie on the frictional rollers. Since no edge gripper is used here, the wafer is quite freely accessible from the front side and the back side, so that basically a simultaneous inspection of both sides is possible. The problem here is that pressing force is limited by the gravity acting on the wafer and therefore large torques cannot be applied. Thus, this type of drive does not provide an adequate rotational velocity. Another problem can be abrasion caused by the direct contact between the moving frictional rollers and the edge of the wafer. Furthermore, in this technique, very high demands are placed on the precision of the device, due to a recess (notch) formed at the edge of the wafer, in order to ensure both an excellent concentric running and a uniform application of torque. For the aforementioned reasons, the invention starts with a different drive principle.
One example of a holding device of this kind is shown in DE-OS 10 2004 036 435 A1. This has a mechanical edge gripper, which supports the wafer largely free of contamination. However, in this process it touches the wafer front side and the wafer back side in its edge region. Furthermore, this holding device can only be used in a narrow range of rpm, because an air pressure gradient is formed above a turntable of the holding device, due to the centrifugal force, and it can warp the wafer.
A holding device is known from U.S. Pat. No. 6,702,302 B2, in which a gas feed is used to adjust pressure conditions in a turntable to ensure a planar support for the wafer. The wafer is fixed in the region of its edges by grippers attached to the table and able to move in the radial direction, and the torque for the rotation of the wafer is also produced by them.
A holding device is known from DE-OS 10 2005 000 665 A1 that likewise uses a cushion of air or gas to hold the wafer at a definite distance in the direction perpendicular to its primary dimensional plane. Moreover, the holding device has nozzles acting at a slant to its plane, producing a force component acting parallel to this plane and fixing the wafer in the radial direction with the help of passive stops. The wafer is placed in rotation by form fit, thanks to a driver engaging with the notch at the edge of the wafer.
Although the last mentioned holders are suitable for high rotational velocities and sometimes also offer an adequate plane running, only one side of the wafer can be inspected, namely, the one opposite the edge grippers, and the wafer has to be turned over to inspect the other side (back side).
A transport device is known from WO 03/060961 A1 for the non-contact supporting and non-contact transport of flat objects, which create a cushion of air by means of openings made in a table forming vent nozzles on the one hand and suction nozzles on the other, which can be adjusted very specifically and which can be used to achieve a uniform supporting and/or a transport of the flat object. In theory, an overhead support is also possible with this, so that the wafer does not need to be turned over, but instead the support system can be brought up from the other side of the wafer.
Moreover, a holding device for the non-contact positioning of flat objects is known from WO 00/61474, which produces levitation sound waves that define a holding plane, in which the object can be suspended, by means of selected energy nodal points.
The aforementioned prior art is not and cannot be associated with a rotational drive in some respects, and in others can only be so associated with great expense. In particular, the distance positioning devices which work by means of fluid flow have the drawback that the supply and drain lines for the fluid require a large construction expense and constitute large moving masses. Such holding devices are therefore unsuited in particular for compact inspection gear that can be integrated into process machinery. Furthermore, they have the drawback that access to the back side of the wafer for scattered light inspection is difficult owing to their construction.
Accordingly, the problem of the present invention is to provide an especially compact holding device with which a flat object, especially a wafer, can be supported with minimal contamination of both sides of the wafer, at high rpm and with planar running of a few thousandths of a millimeter, so that the greatest possible access exists to its front and back side. Furthermore, the holding device should be able to work overhead, so that the wafer can be supported from above and from below and be inspected without having to turn it over.
The problem is solved according to the invention by a holding device with a distance positioning device which is arranged for holding the object perpendicular to the object plane at a defined distance, a lateral positioning device, arranged for positioning the object in the object plane and for rotating together with the object about a rotational axis perpendicular to the object plane, and with a rotational drive, coupled with the lateral positioning device, providing a driving force for rotating the object about the rotational axis, wherein the driving force can be applied to the object by means of the lateral positioning device, wherein the lateral positioning device does not make contact with the object at either its top side or bottom side and the distance positioning device has means for holding the object without involving contact, and is decoupled from the rotational drive in such a way that the distance positioning device does not rotate together with the object. Advantageous modifications are contained in the subclaims.
The invention specifies, in a holding device of the kind mentioned in the introduction, that the lateral positioning device does not make contact with the object at either its top side or bottom side and the distance positioning device has means for holding the object without involving contact, and is decoupled from the rotational drive in such a way that the distance positioning device does not rotate together with the object.
In this way, the rotating part, namely, the lateral positioning device along with the rotational drive, can be simple in design and the moving masses can be kept low. The distance positioning device is stationary relative to it and thus can be adapted with allowance for the aerodynamical conditions quite easily to the available space, especially in consideration of the space needed for the scattered light inspection. Because the lateral positioning device does not touch the object either on its top side or its bottom side, but only in its marginal region, and because the distance positioning device has means for holding the object without involving contact, the danger of contamination of the top and bottom side of the object or wafer is minimized. The object does not have to lie by its flat surfaces on or against a part of the holding device either to ensure the desired distance or for lateral positioning or applying of a driving force.
Preferably, the distance positioning device has a nozzle arrangement, which is designed to create a fluid flow directed essentially perpendicular to the surfaces, which is used to support the object at a defined distance.
Such familiar “Bernoulli chucks” in a stationary arrangement, however, open up the prospect of a simple fluid feeding without the familiar expensive rotary transmission feedthroughs, which additionally increase the moving masses and are critical in regard to maintaining air purity. A fluid flow directed essentially perpendicular onto the surface means that the sum of all fluid flows (when there are several nozzles in the layout) produces no significant lateral force component. Accordingly, only a force which is perpendicular to the surface remains in the sum.
In an alternative advantageous modification, the distance positioning device has sound generating means, which are designed to produce levitating sonic waves.
Such distance positioning devices are also familiar, as mentioned in the introduction, but only for use in technological processes in which horizontal and vertical transporting, warehousing, or temporary storing is required. The use in connection with a rotation, especially for the inspection of the object, is not known. The benefits of this kind of distance positioning lie in a lower consumption of air and energy and consequently easier keeping clean of the air.
In another advantageous configuration, the distance positioning device has evacuation means that are designed to create a local partial vacuum and that interact with the nozzle arrangement or sound generating means to hold the object at the defined distance.
It has been found, especially in comparison to a nozzle arrangement with no evacuation means, that the evacuation means make it possible to lower the volume flow of fluid as well as its flow velocity, which lessens the danger of contamination. In combination with the sound generating means, the additional evacuation means have proven to be beneficial, since such a device is suitable for overhead supporting of the object and distance positioning, so that the wafer's back side can be inspected without having to turn the wafer over.
The distance positioning device according to one advantageous modification has an essentially flat table, in which the nozzle arrangement, the sound generating means, and/or the evacuation means are arranged so that the object is held at the defined distance parallel to the table.
In one advantageous modification, the table has an opening for an optical measurement system to have access to the object.
Since the distance positioning device and thus the table are arranged stationary, the optical measurement system can thus be brought up to the required distance, say, from the bottom through the opening, in order to sense the surface of the object. The opening can be in the form of a through hole, for example, or in the form of a gap dividing the table into two halves, but each time one must make sure that the aerodynamical conditions between the table and the object assure the required distance positioning.
In one advantageous embodiment, the lateral positioning device is designed to apply the driving force to the object by means of a frictional connection. In another embodiment, it can be advantageous for the lateral positioning device to be designed to apply the driving force to the object by means of positive locking.
Especially preferably, the lateral positioning device has deflection means which are designed to permit a deflection movement of the object in the distance direction.
This is advantageous, because otherwise the holding device is mechanically over-determined with respect to the holding force in the distance direction, i.e., perpendicular to the object plane, so that if the axis of rotation is not oriented perfectly perpendicular to the object plane alternating forces may attack the object and can cause it to vibrate or produce particles by abrasion.
The deflection means preferably have spring elements assigned to each of the stop element, edge gripper and/or driver and acting in the distance direction.
In another advantageous modification, for the same reason the lateral positioning device is designed to exert practically no holding force on the object in the distance direction.
The lateral positioning device and the distance positioning device can be arranged on the same side of the object plane. This design is more simple in construction. The lateral positioning device and the distance positioning device can be arranged on the opposite side of the object plane, however, for example, for design-related requirements.